1. Field of the Invention
The present invention relates to a display element capable of displaying multiple colors, a method of writing images to it, and an apparatus for writing images.
2. Description of the Prior Art
A reflective liquid crystal is suitable as a display element of small-size information equipment, portable information terminals and the like because it does not require a dedicated light source such as backlight, has low power consumption, and can be of a thin and lightweight construction.
There is known a reflective liquid crystal element capable of displaying multiple colors that, between a pair of substrates each having an electrode formed on an inner surface thereof, three liquid crystal cells forming display layers having cholesteric liquid crystals selectively reflecting blue, green, and red lights are stacked, and a light absorption layer is formed on the back of a liquid crystal cell opposite to a display side (a side through which outside light comes).
In the cholesteric liquid crystal display element of cell stacking type, by independently having the cholesteric liquid crystals of cells switch between a selective reflection state due to a planar state and a transmission state due to a focal conic state, eight colorsxe2x80x94white, black, blue, green, red, cyan, magenta, and yellowxe2x80x94can be displayed within one pixel, and a display with low loss of light and high contrast can be obtained because no color filter is used.
However, the cholesteric liquid crystal display element of cell stacking type has the disadvantages that parallax becomes high because there are a substrate and an electrode between display layers of different colors and the interval between the display layers becomes large, and the display element and a display apparatus are expensive to fabricate because driving electrodes and driving circuits for three colors are required.
A cholesteric liquid crystal display element capable of displaying multiple colors is proposed in Japanese Published Unexamined Patent Application No. Hei 10-177191 (hereinafter referred to as a first conventional example) and Japanese Published Unexamined Patent Application No. Hei 11-149088 (U.S. patent application Ser. No. 09/192,402, hereinafter referred to as a second conventional example). According to the proposed cholesteric liquid crystal display element, three display layers having cholesteric liquid crystals selectively reflecting blue, green, and red lights are stacked between a pair of substrates each having an electrode formed on an inner surface thereof, and an image is written and displayed by applying a writing signal from the outside of the three display layers.
FIG. 28 shows a first conventional example. In a display element 31 of this example, between a substrate 32 having a writing electrode 34 formed on an inner surface thereof and a substrate 33 having a writing electrode 35 formed on an inner surface thereof, three display layers 38A, 38B, and 38C of PDLC (Polymer Dispersed Liquid Crystal) in which cholesteric liquid crystals 41A, 41B, and 41C selectively reflecting mutually different color lights are respectively droplet-dispersed in polymeric matrix 42 are stacked, and a light absorption layer 39 is formed on the back of a substrate 33 of a non-display side. Threshold voltages of orientation changes of the cholesteric liquid crystals 41A, 41B, and 41C are set as described later. The writing electrodes 34 and 35 are connected to a writing apparatus (driving circuit) 50.
FIG. 29 shows a second conventional example. In the display element 31 of this example, between the substrates 32 and 33, three display layers 38A, 38B, and 38C having the cholesteric liquid crystals 41A, 41B, and 41C selectively reflecting mutually different color lights are stacked in a way that inserts spacers 37A, 37B, and 37C into the display layers 38A, 38B, and 38C, respectively, and puts a separating substrate 36A between the display layers 38A and 38B and a separating substrate 36B between the display layers 38B and 38C, and the light absorption layer 39 is formed on the back of the substrate 33 of the non-display side. Threshold voltages of orientation changes of the cholesteric liquid crystals 41A, 41B, and 41C are set as described later. The writing apparatus 50, which is formed separately from the display element 31, includes the electrodes 54 and 55 sandwiching the display element 31, and a driving circuit 51 for applying a writing signal between the electrodes 54 and 55.
A cholesteric liquid crystal having positive dielectric anisotropy has three states: a planar state in which helical axes are vertical to cell surfaces and which causes a selective reflection phenomenon for incident light, as shown in FIG. 26A; a focal conic state in which helical axes are almost parallel to cell surfaces and which causes incident light to transmit while scattering a little forward, as shown in FIG. 26B; and a homeotropic state in which a helical structure collapses and liquid crystal directors face a field direction and which causes incident light to transmit almost perfectly, as shown in FIG. 26C.
The planar state and the focal conic state of the three states can exist bistably when no electric field is applied. Therefore, the orientation states of cholesteric liquid crystals are not uniquely determined for electric fields; when an initial state is the planar state, as an applied voltage increases, the cholesteric liquid crystals change in the order of the planar, focal conic, and homeotropic states; and when an initial state is the focal state, as an applied voltage increases, the cholesteric liquid crystals change in the order of the focal conic and homeotropic states. On the other hand, if an electric field is suddenly set to zero, the planar and focal conic states remain unchanged, and the homeotropic state changes to the planar state.
Therefore, immediately after a pulse signal is applied, the cholesteric liquid crystal layers exhibit an electo-optical response as shown in FIG. 27; when an applied pulse voltage is Vfh90 or more, it enters a selective reflection state representing a change from the homeotropic state to the planar state; and when an applied pulse voltage is between Vpf10 and Vfh10, it enters a transmission state due to the focal conic state; and when an applied pulse voltage is Vfh90 or less, it maintains the state in which it was before the pulse signal is applied, that is, enters the selective reflection state due to the planar state or the transmission state due to the focal conic state.
In the figure, the vertical axis represents normalized reflectivity and normalizes reflectivity by a maximum reflectivity of 100 and a minimum reflectivity of 0. Since change of reflectivity entails a transition area, a normalized reflectivity of 90 or more is defined as a selective reflection state; a normalized reflectivity of 10 or less, as a transmission state; threshold voltages of change between the planar state and the focal conic state, as Vpf90 before a transition area and Vpf10 after it; and threshold voltages of change between the focal conic state and the homeotropic state, as Vfh10 before a transition area and Vfh90 after it.
In the conventional display element 31 shown in FIGS. 28 and 29, these threshold voltages are mutually changed among the display layers 38A, 38B, and 38C. Specifically, assuming that threshold voltages of the display layer 38A are Vpf90(A), Vpf10(A), Vfh10(A), and Vfh90(A); threshold voltages of the display layer 38B are Vpf90(B), Vpf10(B), Vfh10(B), and Vfh90(B); and threshold voltages of the display layer 38C are Vpf90(C), Vpf10(C), Vfh10(C), and Vfh90(C), an expression (6) shown below is set.
Vpf90(C) less than Vpf10(C) less than Vpf90(B) less than Vpf10(B) less than Vpf90(A) less than Vpf10(A) less than Vfh10(C) less than Vfh90(C) less than Vfh10(B) less than Vfh90(B) less than Vfh10(A) less than Vfh90(A)xe2x80x83xe2x80x83(1)
The order in which the display layers are stacked is not limited to the examples of FIGS. 28 and 29. That is, regardless of the order in which the display layers are stacked, when the three display layers are defined as 38A, 38B, and 38C in descending order of threshold voltages Vpf90, Vpf10, Vfh10, and Vfh90, arrangements are made so that the following expression is satisfied
Vpf10(A) less than Vfh10(C)xe2x80x83xe2x80x83(1a)
and there are no other threshold voltages between both.
When there are a refresh period Tr, a select period Ts, and a following non-voltage display period Td as shown in FIG. 31, by the writing apparatus 50, a writing signal representing a voltage selected from the above-described seven voltage levels Va to Vg demarcated by the threshold voltages as shown in FIG. 30, based on input image data, is applied between the writing electrodes 34 and 35 or between the electrodes 54 and 55, holding the relation that a voltage Vr in the refresh period Tr is greater than a voltage Vs in the select period Ts.
FIG. 32 shows, in this case, the orientation states of the display layers 38A, 38B, and 38C by combinations of refresh voltage Vr and select voltage Vs, wherein xe2x80x9cpxe2x80x9d designates a selective reflection state due to a planar state; xe2x80x9cfxe2x80x9d, a transmission state due to a focal conic state; and xe2x80x9c?xe2x80x9d,an undecided state depending on a state before a write signal is applied. The orientation states indicate the display layers 38C, 38B, and 38A from the left in that order.
As is apparent from the above, according to the conventional display element 31, the following seven types of orientation states are obtained.
(1) All of the display layers 38A, 38B, and 38C are the planar state.
(2) All of the display layers 38A, 38B, and 38C are the focal conic state.
(3) The display layer 38A is the planar state, and the display layers 38B and 38C are the focal conic state.
(4) The display layer 38B is the planar state, and the display layers 38A and 38C are the focal conic state.
(5) The display layer 38C is the planar state, and the display layers 38A and 38B are the focal conic state.
(6) The display layers 38A and 38B are the planar state, and the display layer 38C is the focal conic state.
(7) The display layers 38B and 38C are the planar state, and the display layer 38A is the focal conic state.
Therefore, for example, on the assumption that the display layers 38A, 38B, and 38C selectively reflect blue light, green light, and red light, respectively, as shown in FIG. 32, the display element can assume the following seven display states, so that the five colors of white, black, blue, green and red, and the two colors of cyan and yellow, or seven colors in total can be displayed within one pixel.
(1) White is displayed by a writing signal satisfying relations of Vr=Vg and Vs=Va.
(2) Black is displayed by, e.g., a writing signal satisfying relations of Vr=Vd and Vs=Va.
(3) Blue is displayed by a writing signal satisfying relations of Vr=Vg and Vs=Vc.
(4) Green is displayed by a writing signal satisfying relations of Vr=Vf and Vs=Vb.
(5) Red is displayed by, e.g., a writing signal satisfying relations of Vr=Ve and Vs=Va.
(6) Cyan is displayed by a writing signal satisfying relations of Vr=Vg and Vs=Vb.
(7) Yellow is displayed by, e.g., a writing signal satisfying relations of Vr=Vf and Vs=Va.
In the above example, assuming that the display layer 38B having intermediate threshold voltages selectively reflects green light, cyan and yellow are displayed as two colors of cyan, yellow, and magenta. However, if it is assumed that the display layer 38B having intermediate threshold voltages selectively reflects blue light, cyan and magenta can be displayed as two colors of cyan, yellow, and magenta, and if it is assumed that the display layer 38B having intermediate threshold voltages selectively reflects red light, yellow and magenta can be displayed as two colors of cyan, yellow, and magenta.
In the conventional display element 31, except for the thin separating substrates 36A and 36B of the example of FIG. 29, no substrate and electrode are provided between the display layers 38A, 38B, and 38C so that the intervals between the display layers 38A, 38B, and 38C become zero or very small, with the result of low parallax and reduced costs of fabricating the display element and display apparatus because the writing electrodes and the driving circuit are made common among the display layers 38A, 38B, and 38C.
However, the above-described conventional display element 31 has the disadvantages that combinations of orientation states of cholesteric liquid crystals 41A, 41B, and 41C of the display layers 38A, 38B, and 38C, determined by refresh voltage Vr and select voltage Vs, are no more than seven types, and the five colors of white, black, blue, green and red, and two colors of cyan, yellow, and magenta, which are determined by the relationship between the magnitude of threshold voltages of the cholesteric liquid crystals 41A, 41B, and 41C and selectively reflected colors, that is, no more than seven colors in total can be displayed, indicating a narrow color reproduction area (color reproduction range).
Accordingly, according to the present invention, in a display element in which three or more display layers for displaying mutually different color lights are stacked within one pixel and which controls display states of the display layers by applying a voltage from the outside of the display layers, eight colorsxe2x80x94white, black, blue, green, red, cyan, magenta, and yellowxe2x80x94can be displayed within one pixel, and a color reproduction area can be enlarged.
An aspect of the present invention relates to a display element having three or more display layers each including cholesteric liquid crystal. The display layers selectively reflect lights of different peak wavelengths, respectively. The layers are stacked within one pixel and have a threshold voltage of orientation change of the cholesteric liquid crystals differing from each other for voltage applied from the outside of the plural display layers. Among the three or more display layers, a threshold voltage of change from a planar state to a focal conic state of the display layer having the highest threshold voltage is higher than a threshold voltage of change from a focal conic state to a homeotropic state of the display layer having the lowest threshold voltage.
Another aspect of the present invention relates to a method of writing an image to the display element of the present invention by applying a writing signal which includes at least a refresh period, a select period, and a following non-voltage display period. A voltage Vr in the refresh period is greater than a voltage Vs in the select period.
Another aspect of the present invention relates to a display element having three or more display layers including cholesteric liquid crystals selectively absorbing lights of different peak wavelengths, respectively, by adding dichroic dyes to the cholesteric liquid crystals or by the dichroism of the cholesteric liquid crystals themselves. The display layers are stacked within one pixel, and have a threshold voltage of orientation change of the cholesteric liquid crystals differing from each other for voltage applied from the outside of the display layers. Among the three or more display layers, a threshold voltage of change from a planar state to a focal conic state of the display layer having the highest threshold voltage is higher than a threshold voltage of change from a focal conic state to a homeotropic state of the display layer having the lowest threshold voltage. In the display element of the present invention configured as described above, for voltages applied from the outside of the plural stacked display layers, a threshold voltage Vpf90(A) of change from a planar state to a focal conic state of the display layer A having the highest threshold voltage of the three or more display layers A, B, C, . . . is higher than a threshold voltage Vfh90(C) of change from a focal conic state to a homeotropic state of the display layer C having the lowest threshold voltage.
For this reason, by applying a voltage between the two threshold voltages Vpf90(A) and Vfh90(C) to the whole of the plural stacked display layers, orientation states not found in conventional display elements with threshold voltages set as shown in FIG. 30 are obtained so that the display layer A having the highest threshold voltage of the display layers A, B, C, . . . and the display layer C having the smallest threshold voltage go to a planar state and the display layer B having intermediate threshold voltages goes to a focal conic state, so that eight colorsxe2x80x94white, black, blue, green, red, cyan, magenta, and yellowxe2x80x94can be displayed within one pixel, and a color reproduction area can be enlarged.